First no one ever said all the rules had to be fair
Does it matter that B1 is only trying to draw the foul? He has met the rule. He had established legal guarding position and thus is playing GOOD Defense (unless of course you want to get in the discussion if he had set up under the basket but weve been down that road) He has used his quickness to actively defend. He got in front of him in legal guarding position... A has two choices go around him or foul, not my fault A is too slow to go around. B forced the foul, and the turnover. There are no factors that play in the judgement. If In my judgement he has established legal guarding position the offense gets the foul. Why bail the offense out here? I will make the PC call on tis one no doubt.
This isnt any different that setting legal moving screens, setting the blind side screen with the appropriate distance, or one of a myriad of actions that teams use to get a team to violate or foul.
If someone sets a pick and gives the player the one or two steps and then has a violent collision then the foul must be on the screener because it just seems unfair that the screener could cause such a violent collision without penalty.
It seems unfair to call a T when a player only swings his elbows with no contact.